1. Field of the Invention
This invention relates to a power steering device for vehicles.
2. Description of the Prior Art
In a conventional power steering device for vehicles, the capacity of a hydraulic pump is set so that pressure oil can be supplied to a power cylinder at a sufficiently high flow rate while the pump is operated with the engine idling. While the vehicle runs at a high speed or straight forward, i.e., while power assistance is not required, engine power is consumed wastefully due to the pumping loss of the hydraulic pump which has a comparatively large capacity.
With a view to solving this problem, (a) a power steering device in which a clutch or a speed change gear is provided between a pump and a pulley to control the discharge rate of the pump, (b) a power steering device in which a pump is actuated by an electric motor to control the discharge rate thereof, (c) a power steering device in which two pumps of different capacities are used each in its proper way, and (d) a power steering device having a pump of a variable capacity, have already been proposed or practically used. However, the above power steering devices (a)-(c) force the structure of a conventional pump, when used therein, to be greatly changed. This increases the manufacturing cost and the weight of the power steering device. In the above power steering device (d), a decrease in friction loss is unlikely expected.